2. GALAXY MORPHOLOGY

Morphology is the study of the basic patterns of things
(Zwicky 1957).
It is convenient to divide these into patterns of observed
forms and patterns of behavior.

A natural first step in the study of galaxies was to classify them
into a small number of "natural groups"
(Morgan 1951),
each of which
isolates common structural features seen in photographs. Such descriptive
taxonomies are useful because galaxies with similar appearances
turn out to have similar physical properties. This work has led to
well established schemes of galaxy classification
(section 2.2), which have
been successful in describing galaxy physics in two different ways.
First, the classification cells rank galaxies by various physical
parameters, thereby providing insight into galaxy evolution
(section 2.3).
Second, they partially isolate different physical processes. The
development of galaxy morphology consists of revisions and refinements
intended to improve these two types of result. The ultimate aim is to
construct a physical taxonomy, i.e., a categorization by formation
mechanism, physical structure and content. One approach to the eventual
construction of such a physical morphology is discussed in
section 2.4.

Morphological studies of the patterns of galaxy behavior (e.g.,
section 2.5) lead to results of two
distinct kinds. Certain specialized
questions can be answered definitely. One example is the study of galaxy
shapes, including the deduction that many unbarred disks are globally
oval. However, morphology is more generally a "soft" science, which
is best viewed as preparation for more quantitative work. Its most
important use may be to provide a list of specific questions which
provide direction for this work. It is difficult to overestimate the
importance of asking the proper questions, which routinely (if
non-trivially) lead to concrete results even for recalcitrant problems.
This section and section 5 illustrate the
use of morphology both to reach
specific conclusions and to establish questions for quantitative work.